Diesel oxidation catalysts (DOC) are designed to promote chemical oxidation of carbon monoxide (CO) and hydrocarbons (HC) and the soluble organic fraction (SOF) portion of particulate matter (PM). Additional benefits include oxidation of several non-regulated, HC-derived emissions, such as aldehydes or Polycyclic Aromatic Hydrocarbons (PAHs), as well as the reduction or elimination of the odour of diesel exhaust. HCs are oxidised to form carbon dioxide (CO2) and water vapour and CO is oxidised to CO2. Generally, a DOC comprises an active platinum group metal (PGM), typically platinum.
A DOC comprising chromia, palladium (Pd) and platinum (Pt) is known from U.S. Pat. No. 4,303,552.
Some of the oxidation reactions which may occur over a DOC can produce undesirable products and, in effect, act counterproductively to the catalyst purpose. Sulfur dioxide (SO2) can be oxidised to sulfur trioxide (SO3) which can combine with water vapour to form gaseous sulfuric acid H2SO4. Sulfuric acid vapour can combine with further water vapour to generate sulfuric acid particles which are detected as particulates in assessing total PM emissions from an engine. Sulfur can also poison the oxidation activity of the DOC, and it is believed that this is a significant reason why palladium catalysts have not been more widely accepted in the market.
Exhaust gas temperatures for compression ignition engines, particularly diesel engines for light-duty diesel vehicles (as defined by the relevant legislation), are relatively low, e.g. about 300° C. and so one challenge facing catalyst developers is to develop durable catalyst formulations with low-light off temperatures. Although sulfur derived from fuel is to be reduced in the European Union countries (from Jan. 1, 2005, the maximum sulfur content in Euro 4 (type approval) diesel fuel will be 50 ppm, down from 350 ppm, and this level will probably be reduced to 10 ppm by 2010), in the USA levels are not due to be reduced from their present level of 350 ppm until 2007.
In order to meet existing and future vehicular emission standards for particulate matter, it has been suggested to fit a particulate filter in the exhaust system of a vehicle powered with a compression ignition engine. Suitable filter substrates can include ceramic wall-flow filters and sintered metal filters. It is also known to catalyse the filter in order to bring the combustion temperature of the particulate matter down so that it is within, or closer to, the exhaust gas temperatures generated during normal driving conditions. However, exhaust gas temperatures for light duty compression ignition engines are generally too low for a catalysed filter to regenerate passively and so it has been suggested actively to regenerate the filter in an exotherm generated by combusting hydrocarbon fuel on the catalysed filter, or a separate catalyst upstream of the filter. Such an arrangement is described, for example, in GB-A-2064983.
U.S. Pat. No. 4,686,827 describes an exhaust system for a diesel engine in which an electrically heated catalyst is used to generate an exotherm from hydrocarbon fuel injected into the exhaust gas for active regeneration of a downstream filter. In one embodiment, the electrically heated catalyst is platinum-palladium.
In our WO 2004/025093 we describe a compression ignition engine operable in a first, normal running mode and a second mode producing exhaust gas comprising an increased level of CO relative to the first mode and means when in use to switch engine operation between the two modes, the engine comprising an exhaust system comprising a supported Pd catalyst associated with at least one base metal promoter and an optionally supported Pt catalyst associated with and/or downstream of the catalyst wherein CO is oxidised by the supported Pd catalyst during second mode operation. According to the disclosure, increased CO can be generated: by injecting HC into the exhaust system over a partial oxidation catalyst; by adjusting the ignition timing of at least one engine cylinder; and/or adjusting the engine air-to-fuel ratio of at least one engine cylinder. In one embodiment, the Pd catalyst and associated base metal and optional Pt catalyst components comprise a diesel oxidation catalyst.
US 2002/0053202 discloses a combined H2 supplying and SOF adsorbing-oxidising catalyst comprising at least one of Pt, Pd and/or rhodium (Rh) and cerium (Ce)
for use in combination with a downstream NOx absorbing catalyst and periodic enrichment of the air/fuel ratio of the exhaust gas. The Examples illustrate a H2 supplying and SOF adsorbing-oxidising catalyst consisting of Pt/CeO2 and Pt/La.SiO2.